Introduction: Correct rotational alignment of the femoral component is an important factor for successful total knee arthroplasty. This study evaluated relationship between the transepicondylar axis and the posterior condylar axis in normal, varus, and valgus knees.

Methods: Thirty normal knees (mean age: 66.2 years), 30 osteoarthritic knees with varus deformity (67.9 years), and 25 osteoarthritic knees with valgus deformity (70.7 years) were evaluated using magnetic resonance imaging. Femo-rotibial angle on standing anteroposterior radiograph was 185° in the varus knees and 166.1° in the valgus knees. In the transverse view, the angle between the transepicondylar axis and the posterior condylar axis, and the angle between the line perpendicular to the anteroposterior (AP) axis and the posterior condylar axis were measured in each group.

Results: Transepicondylar line showed 6.4° of external rotation in the normal knees and 6.1 of external rotation in the varus knees relative to the posterior condylar axis. However, transepicondylar axis of the valgus knee showed 11.6° of external rotation. This angle was significantly larger than that of normal knee and varus knee (p < 0.05). The line perpendicular to the AP axis was externally rotated from the posterior condylar axis in 6.3° in the normal knees, 6.6° in the varus knees, and 8.8° in the valgus knees. The external rotational angle in the valgus knees was significantly larger than that of the normal and varus knees (p < 0.05).

Discussion and conclusion: These results suggest that there is no hypoplasia of the posterior part of the medial condyle in varus knees, however, posterior part of the lateral condyle in valgus knee is severely distorted. Based on the results of this study, 3 to 5 degrees of external rotation relative to the posterior condyles is not large enough to achieve correct rotational alignment for valgus knees.